An inkjet recording method in which printing is carried out by ejecting ink on a recording medium to form recording dots occupies the interest as a non-impact recording method in which colorization is easy, and recording can be conducted directly on plain paper, and a variety of printers utilizing this system are put into practical use. The inkjet recording method includes an on-demand injection system and a continuous injection system and is described in, for example, Takeshi Agui, et al., Real Color Hard Copy, published by Sangyo Tosho Co., Ltd. (1993), Shin Ohno, Non-Impact Printing—Technologies and Materials—, published by CMC Publishing Co., Ltd. (1986), and Takeshi Amari, Inkjet Printer—Technologies and Materials—, published by CMC Publishing Co., Ltd. (1998). Further, the continuous type includes a recording system called an electrostatic system (for example, Sweet type and Hertz type); and the on-demand type includes recording system called a piezoelectric system, a shear mode piezoelectric system, and a thermal inkjet system.
As the on-demand type inkjet recording method, there are known systems called electrostatic acceleration type ink-jet or slit jet as described in, for example, Susumu Ichinose and Yuji Ohba, Denshi Tsushin Gakkai Rombunshi, Vol. J66-C (No. 1), page 47 (1983) and Tadayoshi Ohno and Mamoru Mizuguchi, Gazo Denshi Gakkaishi, Vol. 10 (No. 3), page 157 (1981). According to this system, a voltage is applied to a plurality of recording electrodes disposed opposite to a recording medium and counter electrodes disposed on the back side of the recording medium, an electrostatic force is allowed to act on ink supplied on the recording electrodes due to a potential difference generated between the both electrodes, thereby ejecting the ink on the recording medium. Specific embodiments thereof are disclosed in, for example, JP-A-56-170, JP-A-56-4467, and JP-A-57-151374. In this system, a long and narrow slit-like ink discharge port having a plurality of recording electrodes in the inner wall thereof is used in place of a nozzle in the conventional inkjet head, ink is supplied into this slit-like ink chamber, and a high voltage is selectively applied to these electrodes, thereby injecting the ink in the vicinity of the electrode to recording paper closely positioned to the slit, thereby conducting recording.
For those reasons, there is no fear as to ink clogging, and the constitution of the head is simple so that a reduction in manufacturing costs can be expected. Further, this system is a method useful for realizing a so-called longitudinal long head having a length such that the width direction of the recording medium can be covered over a wide range.
Examples of drop-on-demand type full-color recording head constituted of such an electrostatic acceleration type inkjet system are disclosed and reported in, for example, JP-A-58-215253 and Denshi Tsushin Gakkai Rombunshi, Vol. J68-C, 2, pp. 93–100 (1985).
In this electrostatic acceleration type inkjet head, oily inks having a dyestuff dissolved in an organic solvent are suitably used. Though constitutional materials of such inks are not disclosed in detail, according to examples seen in Denshi Tsushin Gakkai Rombunshi, Vol. J68-C, 2, pp. 93–100 (1985), inks having such physical property values that the volume resistivity (electric resistivity) is from 107 to 108 Ω·cm, the surface tension is 22 mN/m, and the viscosity of from 3.1 to 6.9 cP are used.
However, these oily inks have a low surface tension as compared with aqueous inks generally used in other ink-jet systems so that their permeability into recording paper is very large. Accordingly, in particular, there was encountered such a problem that in the case where printing is carried out on plain paper, a lowering in the printing density, blur, and offset are liable to occur.
On the other hand, an electrostatic system of a color material concentration discharge type without using a slit-like recording head is disclosed in JP-A-9-193389 and JP-A-138493. In this system, a plurality of individual electrodes for allowing an electrostatic force to act on a colorant component in ink are constituted of a control electrode substrate composed of an insulating substrate having a through-hole formed therein and a control electrode formed corresponding to the through-hole and a convex ink guide arranged in the substantially center position of the through-hole, the ink is carried on the surface of the convex ink guide to an ink droplet ejecting position by a surface tension, and a prescribed voltage is applied to the control electrode to eject ink droplets to a recording medium, thereby conducting recording.
In the electrostatic inkjet system of a color material concentration discharge type, particles of a color material are concentrated into the discharge section by electrophoresis, thereby ejecting the ink droplets in the state that the color material is concentrated in a high concentration. For that reason, different from the foregoing systems, in this electrostatic inkjet systems, the ink is not discharged in the state that a large quantity of the liquid component wherein constitutional components of the ink are uniformly present is contained but is discharged in the state that a mall quantity of the liquid component wherein the color material is agglomerated is contained, whereby the foregoing problems are solved. Also, by using a pigment as the color material, advantageous results are obtained with respect to water resistance and light fastness of printed images as compared with the conventional inkjet heads using a dyestuff.
In the electrostatic inkjet system of a color material concentration discharge type, for the sake of obtaining good printing characteristics such that the printing density is high and that blur and offset are not caused, first of all, it is required that the volume resistivity of ink be sufficiently large. In this way, it bees possible to allow an electric field applied to ink formed by recording electrodes and counter electrodes to reach particles of a color material. When the volume resistivity of the ink is low, the ink is subjected to charge injection by a voltage applied from the recording electrodes and electrically charged. Thus, a tendency that the ink is discharged in the state that a large quantity of the liquid component is contained due to an electrostatic repulsion becomes strong.
Next, it is required that the particles of color material be concentrated into a discharge section at a sufficient speed by electrophoresis. Accordingly, the particles of color material are required to have a sufficient charge amount, i.e., the particles of color material must have a particle electric conductivity with high positive polarity or negative polarity. Moreover, in view of the discharge performance and sedimentation prevention of the particles of color material, it is considered preferable that the particles of color material have a mean particle size of from about 0.1 to 4 μm.
Concretely, for example, JP-A-9-193389 discloses use of ink adjusted so as to have an electric resistivity of 108 Ω·cm or more by dispersing particles of a developer (particles of a solid resin containing at least a colorant component) in a dielectric liquid having an electric resistivity of 1010 Ω·cm or more, the particles of developer having a zeta potential against the dielectric liquid of 60 mV or more and a mean particle size falling within the range of from 0.01 to 5 μm. Such ink can be, for example, prepared by preparing a color material dispersion using a system wherein a hydrocarbon based solvent having a volume resistivity of 1010 Ω·cm or more is used as an insulating liquid, carbon black or an organic pigment is used as the color material, a pigment is contained in or on the surface of a binder composed of a resin or wax, and a dispersant and a charge control agent such as a metallic soap are additionally added. However, JP-A-9-193389 does not disclose in detail the formation of the ink and the preparation method and adjustment method of physical property values of the ink. Also, there was encountered such a problem that it is difficult to hold the charge of the particles of color material stably over a long period of time.
Also, JP-A-2001-139856 discloses an electrostatic inkjet head ink containing a highly electrically insulating dispersion medium of an aliphatic hydrocarbon based solvent system, a color material insoluble in the dispersion medium, and an acrylic acid ester polymer soluble in the dispersion medium and a metallic soap as charge control agents and a method of controlling charge of a color material. According to JP-A-2001-139856, it is possible to impart a high zeta potential to the particles of color material by containing a combination of an acrylic acid ester polymer soluble in the dispersion medium and a metallic soap. However, there was encountered such a problem that it is difficult to hold the charge of particles of a color material (such as carbon black and organic pigments) stably over a long period of time. Though the mechanism of charge generation against carbon black or organic pigments dispersed in the aliphatic hydrocarbon based solvent is unclear in many points, it may be considered that in the case of a metallic soap, polarity control of the pigment is determined depending upon a selective adsorption model. That is, the oil-soluble metallic soap is dissociated, and the dissociated metal ion adsorbs on the pigment particle to give a positive charge. On the other hand, a counter ion forms a micellar structure together with the metallic soap, whereby it is stabilized an a negative charge in the dispersion. However, there was encountered such a problem that because of the charge generation mechanism based on adsorption equilibrium, it is likely affected by the environment such as water so that it is difficult to hold the charge of the pigment particles stably over a long period of time.